Wire electrode protection system for electrical discharge machining

ABSTRACT

A wire feed protection system for an electrical discharge machining apparatus and machine tool. The workpiece is supported for movement along two coordinate axes in the &#34;X&#34; and &#34;Y&#34; direction through electrically controlled stepping motors in a manner which is well known to the art. The electrode is of a wire or band configuration and is so supported, guided, and driven that the cutting portion of the electrode which is aligned with the workpiece before and during cutting remains properly oriented and travels without bend or distortion in its path. When the wire breaks, bows or otherwise is caused to lose tension, a switch is operated to interrupt machining power pulses being supplied to the machining gap. For this purpose, a switch is located at one or more critical and predetermined locations in the wire electrode path, with the switch actuator in each case in engagement with the wire during transversal.

REFERENCE TO RELATED APPLICATIONS

This application is related to the following copending applications, allof common ownership with the present application:

    ______________________________________                                        Serial                                                                              Filing                                                                  No.   Date     Title                                                          ______________________________________                                        532,200                                                                             12-12-74 Wire Electrode Feed System For Elec-                                          trical Discharge Machining                                                    Inventor: Frank P. Rietveld                                    532,199                                                                             12-12-74 Wire Electrode Feed And Cut-Off System                                        For Electrical Discharge Machining                                            Inventor: Frank P. Rietveld                                    534,292                                                                             12-19-74 Electronic Alignment System For A                                             Wire Electrode                                                                Inventors: Randall C. Gilleland and                                           Frank P. Rietveld                                              551,957                                                                              2-21-75 Servo Feed System For A                                                       Wire Electrode Type Electrical                                                Discharge Machining Apparatus                                                 Inventors: Oliver A. Bell, Jr. and                                            Randall C. Gilleland                                           543,989                                                                              1-27-75 Electrical Discharge Machining Device                                         For Cutting With Wire Electrode                                               Inventor: Frank P. Rietveld                                    555,821                                                                              3- 6-75 Constant Wire Feed Control System For                                         Electrical Discharge Machining                                                Inventors: Oliver A. Bell, Jr. and -  Randall C.                              Gilleland                                                      545,929                                                                              1-31-75 Electrical Discharge Machining Device                                         For Taper Cutting With Wire Electrode                                         Inventor: Frank P. Rietveld                                    567,345                                                                              4-11-75 Servo Feed System For                                                         Through Hole Cutting For Elec-                                                trical Discharge Machining Apparatus                                          Inventors: Oliver A. Bell, Jr. and                                            Randall C. Gilleland                                           588,060                                                                              6-18-75 Servo Feed System For                                                         Through Hole Machining For Elec-                                              trical Discharge Machining Apparatus                                          Inventors: Oliver A. Bell, Jr. and                                            Randall C. Gilleland                                           583,794                                                                              6- 4-75 Programmable Current Control System For                                       Wire Electrode Electrical Discharge                                           Machining Apparatus                                                           Inventors: Oliver A. Bell, Jr. and                                            Randall C. Gilleland                                           ______________________________________                                    

BACKGROUND OF THE INVENTION

The present invention relates to an improved wire feed system forelectrical discharge machining. The system in many cases eliminates thenecessity of relatively expensive three dimensional electrical dischargemachining electrodes that must be machined by precision machiningoperations, which greatly add to the cost of the operation. The wirefeed system by its use of a precisely driven and guided wire electrodemakes possible the generation of very small radii in intricate shapes.As the wire electrode process has expanded to press tooling, extrusiondies, powdered metal dies and precision prototype parts, it becamenecessary to improve the machine tool head and to provide a wire feedsystem design which is specifically made for wire feed and operable toaccommodate the necessary standards of reliability and performance. Ithas been necessary to completely redesign the head assembly of thestandard EDM machine tool to provide for the wire feed system.

It will be understood that the present invention as it relates to themachine tool portion of the EDM apparatus is used in conjunction with anEDM power supply which comprises a pulse generator that providesmachining power pulses to the machining gap. The machining power pulsesare of precisely controllable on-off time and current magnitudetherefore to control the rate of machining, the finish of the workpieceand the degree of the overcut.

In the use of a wire electrode, the level at which machining isaccomplished is typically of a high voltage level, which may range asmuch as 300 volts. A typical range of machining current would be 15amperes of cutting current either infinitely variable or with as many asten steps of current limiting provided.

It will be understood that the tables on a typical two axis contouringmodel, such as is shown in the drawings, would be capable of performingboth straight line motions and continuous path contouring. Any of anumber of commercial numerical tape control units are available forproviding this control. One example of such a unit is the numerical tapecontrol unit manufactured by the Superior Electric Company of Bristol,Connecticut.

The prior art has shown a variety of wire and band electrode feedarrangements which are adapted to provide the type of machining withwhich we are currently concerned. However, certain difficulties havearisen in the use of such systems. Among the major problems are therequirement for causing a length of wire to pass with a constantvelocity through the machining gap region. This constant velocitymovement is necessary since the cross section of the wire changes duringmachining. Unless the movement of the wire through the gap is maintainedat a constant velocity, the machining slot would be of a varying width.This condition could not be tolerated in any precision machiningoperation.

An example of an early type of band electrode machining arrangement isshown and described in U.S. Pat. No. 2,903,557 issued on Sept. 8, 1959to V. E. Matulaitis for "Arc Machining With Band Electrode". In thatpatent, the problem of maintaining a constant width band or wireelectrode was handled by providing a feed of the electrode with avelocity that was regulated in accordance with a feed-back signalrepresentative of average current through the gap. The system forguiding and driving the band electrode consisted of a pair of reels,that is, a supply reel and a windup reel. In the two reel system, it wasexceedingly difficult to avoid electrode bend or bowing in the wirebetween the two reels.

Other prior art arrangements are shown and described in U.S. Pat. No.3,731,043 issued on May 1, 1973 to Ullmann et al. for "Digital Circuitfor an Eroding Machine" and U.S. Pat. No. 3,636,296 issued on Jan. 18,1972 to G. G. Semin for "Arrangement for Stopping Automatically anElectric Spark Eroding Machine Having Its Work-Performing Electrode Toolin the Shape of a Wire or Band". Both of these patents show systemssimilar to the Matulaitis patent in that the wire or band electrode atits ends is supported and unreeled from spools and reeled onto spools.Such an arrangement has in it the difficulty of providing frequent andunpredictable bends with accompanying changes in velocity of theelectrode band or wire such that the gap being machined varies inthickness. Accordingly, the overcut and the dimensions of the machinedarticle cannot ordinarily be maintained within the necessary limits.

An additional shortcoming not overcome by the prior art is the problemof maintaining that section of the wire electrode which is doing thecutting, that is in the region proximate to the workpiece, in a fixed,properly oriented path to achieve the required cut. Typically, if thecontrol exercised is a two dimensional control with an X and a Y axistable, it would be necessary to maintain the wire with its operativecutting length perpendicular to the table carrying the workpiece or,alternately stated, perpendicular to the plane in which the workpiece islying.

The invention of Applicant with respect to the alignment and support ofthe wire in this critical region includes a novel and highly effectivearrangement between two orthogonally arrayed guide rollers, adjustablealong their axes of rotation, such that this precise alignment is notonly obtainable from the very beginning of the cutting operation but iscontinued throughout the cutting cycle. As shown in the aforementionedU.S. Pat. No. 3,731,043, the wire in the critical cutting region ismaintained between simple pins. In the aforementioned U.S. Pat. No.3,636,926, the wire is supported between parallel axis rollers so thatthe major benefits and advantages of a system like Applicants' are notpossible.

SUMMARY OF THE INVENTION

The present invention thus provides an improved driving and feedingarrangement for a wire or band electrode. The system is capable ofdriving the electrode with a constant velocity during the machiningoperation. Just as importantly, the electrode is precisely alignable andmaintainable in position, that is, the operative cutting length of it isproperly aligned to the workpiece, so that the proper linearrelationship between the wire electrode and the workpiece can be held.In the event the wire tension is lost through breakage, bowing, orotherwise, a sensing switch or switches will be actuated to interruptthe machining power pulses and prevent damage to the workpiece until thecondition can be corrected.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in the appended drawings, withlike numerals being assigned to like parts as they may appear throughoutthe several views, and in which:

FIG. 1 is a partial front perspective view of a machine tool forelectrical discharge machining incorporating the present invention;

FIG. 2 is a right side elevational view of the machine tool head of FIG.1;

FIG. 3 is a left side elevational view of the machine tool head of FIG.1 showing the electrical circuit portion of the protection system;

FIG. 4 is a view substantially similar to FIG. 3 but drawn to anenlarged scale to show better the detail of the cut-off Mechanism andthe associated driven and idler roller arrangement; and

FIGS. 5 and 6 are enlarged scale, partially sectional views taken fromthe side and from the front of the machine tool head to betterillustrate the parts used in the wire support and drive system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With more particular reference to FIG. 1, there is shown a machine tool,indicated generally by the numeral 10. The EDM power supply which isassociated with the machine tool 10 is not shown except in a blockdiagrammatic form in FIG. 3. It will be understood that the power supplymay be any of a number of commercial independent pulse generators orrelaxation oscillator type pulse generators as are well known in the EDMart.

The machine tool 10 includes a base 12, which typically has built intoit a dielectric fluid reservoir and dielectric fluid temperature controlunit for handling the dielectric fluid used in the EDM operation. Thedielectric fluid may be either an ionized water supply or, alternately,kerosene or a like light cutting oil. Throughout the cutting operation,a stream of dielectric fluid is continuously directed at the electrodewire and the workpiece to maintain flow through the gap from either orfrom both the upper and lower directions.

A precision workpiece table 14 is included in the machine tool. Thetable 14 typically has a programmable travel table along both the X andthe Y axis. The system for driving the table 14 generally includesprecision preloaded ball bearing lead screws (not shown) which areeffective to eliminate backlash. A four-sided work pan 15 surrounds thework area to contain the dielectric fluid. The table 14 also includes awork holding fixture 16 which is preferably quick clamping and capableof fastening workpieces of various sizes securely in place.

The machine column is indicated generally by the numeral 18, andincludes a height adjustable head 19 which is adjustable through anassociated precision rack and gear arrangement to bring the wire guidesystem close to the workpiece to maintain accuracy.

The wire electrode system includes a supply reel 20 from which isunwound the electrode wire 22. The electrode 22, while it is illustratedas being of a wire configuration, may alternately be a band electrodewith appropriate changes to be made known in the configuration of theassociated guide and drive rollers. Guide rollers 26, 28, 30, 32, 34 areincluded in the system as shown in FIGS. 1 and 2 to carry the electrode22 through a predetermined cutting path. The system further includes avariable tension brake 25 with a precisely controllable tension adjustsystem including switches 25a. A constant tension is maintained on theelectrode wire 22 by the brake 25. The brake 25 is preferably of theadjustable torque, permanent magnet type and movement of the switches25a causes tension adjustment, for example, in the range from 0.1 to 8.0ounces by varying the magnetic flux. In the interest of brevity andsimplification, the full detail of the parts of the brake 25 is notshown.

FIG. 2 shows in greater detail the arrangement of the wire 22 relativeto the tension brake 25. The wire 22 is wound with one or two turnsclockwise or counterclockwise about the capstan 25b of the tension brake25. At the forward end of the head 18, the electrode 22 takes an upwardpath about the periphery of the roller 28, then passing about theperiphery of the roller 30. It is significant that the rollers 28 and 30have their axes of rotation orthogonal one to the other. This serves togreatly improve and insure the alignment accuracy which may be made inthe vertical plane for the portion of the electrode 22 which istypically maintained in cutting proximity to the workpiece 23. Theworkpiece 23 is shown in the position in which it would be maintainedand supported by the table 14.

The electrode 22 next passes about the roller 32 which again has an axisof rotation orthogonal relative to the axis of the roller 30. Also shownin FIG. 2 is the lever 35 which through a precision rack and gearassembly, including pinion gear 36 and rack 38, is used to allowvertical adjustment of the head assembly 19. A locking screw 40 isprovided to hold the head assembly 19 securely in place after its heightadjustment has been selected. Also shown in FIG. 2 is a portion of thespindle assembly 42 which is used to make linear adjustment of theroller 30 along the Y axis. A further spindle is included cooperativewith the roller 28 to provide for its adjustability along the X axis aswill be better shown in FIGS. 5 and 6 hereinafter.

FIG. 3 shows the remainder of the electrode wire support and guidancesystem and the driving roller system therefor. The driving roller systemincludes a lower driven roller 44 and an opposed idler roller 46. Theroller 46 is connected through a suitable intermediate gear mechanism toa lever 48 which is selectively moved to engage or disengage fromoperative position the roller 46. Next, adjacent the rollers 44 and 46,is a forwardly spaced guide roller 50 and a following cutter arrangementincluding an anvil 52 having a central channel 54 bored through it forthreading therethrough the electrode wire 22. The left hand end of theanvil 52 is shaped with a circular or flat cut-out portion 69 toaccommodate a rotary cutter 56. The cutter 56 is rotated at apredetermined rate to provide cut-off of segments from the used wire 22,thus to drop it in easily disposable form into a lower tray orreceptacle 58. It will be seen that the rotary cutter 56 includes anumber of cutting tips 56a which are sequentially engageable with thewire 22 as it exits from the feed system. The adjustment of the cut-offlengths of the wire 22 may be made by suitable adjustment of the speedof rotation of cutter 56. The pair of electrical drive motors associatedwith the cutter 56 and the driving roller 44 may be of the DC type forproviding ready adjustment of speed of rotation of both elements. Withproper synchronization between the two motors, the cut-off of the usedsegments will proceed without interruption of the wire feed.

Also included in the FIG. 3 drawing is a protective system forinterrupting the operation of the wire feed apparatus. The systemincludes an electrical switch 60 having its actuator 62 slidablyabutting with electrode wire 22 as it passes between the roller 50 andthe rollers 44, 46. FIG. 2 shows a like switch 61 near the beginning ofthe wire feed system with its actuator 63 positioned in slidableengagement with wire 22 just beyond the roller 26. Switch 61 has asimilar function to that which will be described for switch 60. In theevent the electrode wire 22 becomes broken or loses tension throughbowing or uneven unwinding from reel 20, the switch actuator 62 will bemoved into position to operate the normally closed switch 60, thus tointerrupt the power from the associated power supply which providesmachining power pulses to the gap. The power supply is shown in FIG. 3in block form identified by the numeral 64. The output from the powersupply 64 is passed to the electrode wire 22 and to the workpiece 23 sothat the necessary machining power pulse initiation occurs as is wellknown in the EDM art. The switch 60 is illustrated as a normally closedswitch and responsive to the movement to the moved position of actuator62 the machining power pulses to the gap will be interrupted. The switch61 may also be included in series with the power supply 64 to provide asimilar disconnect function.

FIG. 4 further shows the means used to provide a rotative drive to thegear cutter 56 and to the driven roller 44. A gear 65 is shown which ispreferably coupled to the output shaft of an electrical drive motor 66.Through intermediate gear trains, not shown, the drive is passed fromthe gear 65 to the rotary cutter 56 and to the driven roller 44.Alternately, a separate drive means, such as a separate DC drive motor,may be provided for both the cutter 56 and for the roller 44 to operatethem in synchronization. The lever 48 is coupled through a suitableintermediate mechanism to the upper idler roller 46 in such manner as topermit its rocking upward disengagement away from the roller 44 so thatthe wire 22 may be threaded into the passage 54 of the anvil block 52.Suitable adjusting screws, such as adjusting screws 68, are included toallow for adjustment of the anvil 52 and its radially curved or flatportion 69 so that the proper cutting relationship may be maintainedbetween the cutting edges 56a and the curved or flat cut-out portion 69to insure complete severance of each section from the wire 22.

FIG. 5 is shown partly in section to clarify the manner in which thehead assembly 19 is vertically adjusted through the operation of thelever 34. The lever 34 rotates the pinion gear 70, which is in mesh withthe rack gear 72 formed on the opposed surface of the head 19. In thismanner, the height of the head 19 may be adjusted when necessary by theoperator. A locking screw 74 is included which may be manually tightenedto hold the head 19 in the selected vertical position.

Also shown in FIG. 5 is a detail of the mechanism used to provideadjustment in the Y axis of the roller 30 which is located proximate theupper surface of the workpiece 23. In the interest of simplicity, theFIG. 1 showing of the two transverse X and Y axial tables is notrepeated. It is, of course, necessary that there be a precise adjustmentin a vertical direction of the wire 22 preliminary to the cuttingoperation. It will be seen also that the roller 30, like the otherrollers used in the wire feed system, includes a V-type cut-out portion30a to suitably retain the wire 22 in place as it passes over theroller. The spindle which mounts the roller 30 for selective adjustmentalong the Y axis is indicated generally by the numeral 42. The spindle42 includes a central housing 78 and a central shaft 80 threaded andadapted to receive at its left hand end a lock nut 82. The guide roller30 and the shaft 80 on which it is rotatably mounted are movablelongitudinally with the housing 78, either to the right or to the left,and when the proper position is selected, the lock nut 82 is tightenedto secure the assembly in place. Also shown is the roller 32 which nextreceives the wire 22 from about the periphery of the roller 30 andpasses it forward in the system as previously illustrated. At the lowerright hand portion of the FIG. 5 drawing, there is shown the roller 28which is mounted in a rotatable manner proximate the lower surface ofthe workpiece 23.

FIG. 6 shows the detail of the manner in which the roller 28, which isdisposed near the lower surface of the workpiece 23, is mounted. Thespindle assembly 42 is substantially the same as the one already shownin FIG. 5 and used in connection with the roller 30. It will thus beseen that the roller 28 is selectively movable leftwardly or rightwardlyalong the X axis so that it is possible to make a precise verticaladjustment of the wire relative to the workpiece 23. It is significantthat the axes on which rollers 28 and 30 are rotatably mountable arepositioned orthogonal one to the other. This positioning greatly assistsin initially setting and in maintaining the vertical alignment which isso critical to accurate machining of the workpiece 23. Also shown inFIG. 6 is a portion of the cartridge 84 which serves as a further guideto the wire 22. The guide roller 34 will be seen to be mounted on ashaft 86 which itself is adjustable leftwardly through the operation ofa threaded adjusting screw 88. The final exiting of the electrode wire22 is between the opposed surfaces of the driving roller 44 and thedriven roller 46 shown in the upper left hand corner of the FIG. 6drawing.

It will thus be seen that the present invention provides a novel andgreatly improved system for mounting and guiding a wire elecrode on amachine tool for electrical discharge machining. The elements used tomaintain the continuous path of the wire electrode relative to theworkpiece are arranged to make its adjustment precise, particularly inthe area of the workpiece opposed portion. The entire system includes asingle reel, such as supply reel 20, and the means for providing aconstant velocity movement of the electrode wire incorporates positivedriving as between the driving and driven roller through an electricalmotor drive. In this way, it is possible to virtually eliminate the wirebend and distortion problem which might interfere with the accuracy ofthe wire cutting EDM operation.

What is claimed is:
 1. An electrical discharge machining apparatus whichprovides machining of a workpiece by means of a conductive wire for theelectrode mounted on a head, comprising;a supply reel for the wiremounted on the head in a freely rotatable manner; a plurality of guiderollers for retaining and transporting said wire in a predetermined pathto provide a cutting pass and translational movement relative to theworkpiece; said guide rollers including a separate pair of orthogonalaxis guide rollers, respectively, positioned proximate opposite sides ofthe workpiece; a drive motor; a pair of opposed rollers, one driven bysaid drive motor and the other idle, biased one against the other toengage and provide a constant and uniform pulling force upon the wire asmachining progresses; and a switching means having its actuator insensing abutment relative to said wire at a point intermediate saidopposed rollers and the last of said guide rollers proximate thereto,said switch operable to interrupt power to said drive motor responsiveto loss of tension of the wire at said point.
 2. The combination as setforth in claim 1 wherein there is provided a separate means operablyconnected to said driven opposed roller for raising it from engagementwith said opposed idle roller to provide rethreading of said electrodewire after breakage thereof.
 3. The combination as set forth in claim 1wherein an independent source of machining power pulses is operablyconnected across the workpiece and electrode gap and wherein saidswitching means is further operably connected to and effective tointerrupt the connection of said machining power source to the gapresponsive to the loss of tension of said wire.
 4. The combination asset forth in claim 1 wherein a second switching means is mounted on saidhead proximate the wire at the point it is being removed from saidsupply reel, said second switching means having its actuator in sensingcontact with said wire for responding to loss of tension of said wireand interrupting said power.